This invention relates to alloy materials, and more specifically to an alloy which is highly abrasive resistant.
An essential component of any steam generation system of the type, which utilizes pulverized coal as a fuel, is the apparatus in which the coal is pulverized so as to render it suitable for such usage. One form of apparatus in particular, which has frequently been used for this purpose, is that commonly referred to as a bowl mill by those in the industry. Two of the essential components of such a bowl mill are the body portion, i.e., housing, within which a grinding table is mounted for rotation, and the plurality of grinding rolls that are supported in equally spaced relation one to another in a manner so as to coact with the grinding table such that the coal disposed on the surface of the grinding table is capable of being ground, i.e., pulverized, by the rolls. For this purpose, the grinding rolls are each designed to be mounted on a shaft-like member whereby the grinding rolls are capable of movement relative to the surface of the grinding table. Accordingly, in order to effect the assembly of the grinding roll with the aforesaid shaft-like member, the former preferably has a through passage provided through the center thereof. This through passage enables the shaft-like member to be positioned therewithin in mounted relation thereto. Therefore, in order to facilitate the task of providing the grinding roll with such a through passage, it is desirable that the grinding roll be made of a material that is characterized by its ease of machinability, i.e., a relatively soft material such as ductile iron.
In contradistinction to the desirability of making the grinding roll from a relatively soft material, there also exists a requirement that at least the external, i.e., outer, surface of the grinding roll be of a material characterized by good wear-resistant qualities. The reason for this is that in the course of effecting the pulverization of material with these grinding rolls, the latter are of necessity subjected to a harsh abrasive action by virtue of the nature of the material being pulverized as well as by virtue of the manner in which the pulverization takes place. The result, therefore, is that the grinding rolls are susceptible to being rendered unusable because of excessive wear in a relatively short period of time, i.e., the rolls are found to have a relatively short operating life. Obviously, such a result is to be avoided, if possible. In this context, it should be noted that the wear which grinding rolls employed in bowl mills actually experience is influenced principally by the grinding characteristics of the material being pulverized as well as by the productive output of the bowl mill, i.e., the volume of material that is being pulverized within the bowl mill in a given period of time.
When the external surface of the pulverizer, i.e., grinding, roll becomes sufficiently worn to preclude any further use thereof for purposes of effecting the pulverization of material therewith, the remaining portions of the roll are normally still functional. That is, but for its worn external surface, the grinding roll would still be capable of being used. Thus, from the standpoint of achieving economies of manufacture, it would be desirable, if possible, to effect a resurfacing of the external surface of the grinding roll such that the latter might once again be utilized for purposes of pulverizing material rather than to have to replace the worn grinding roll with a totally new grinding roll. In this regard, one would normally expect that the cost of reconditioning, i.e., resurfacing, the external surface of a worn grinding roll would be significantly less costly than manufacturing a totally new grinding roll.
Apart from the relative cost of resurfacing the external surface of a worn grinding roll vis-a-vis that of manufacturing a totally new grinding roll, there is also the matter of the operating life achievable with a resurfaced worn grinding roll as contrasted to that obtainable with a new grinding roll. That is, in order to be economically feasible, it is desirable that the operating life of a resurfaced worn grinding roll be approximately equivalent to or greater than that which one would expect from a new grinding roll. Namely, any savings achievable from utilizing a resurfaced worn grinding roll as opposed to employing a new grinding roll should not be dissipated by virtue of the fact that the operating life of the former is such that several resurfacings thereof would be required in order to achieve an operating life with the worn grinding roll, which is commensurate with that obtainable from the use of a totally new grinding roll. In summary, if in the interest of obtaining comparable operating lives, multiple resurfacings of the worn grinding roll would be required, the cost thereof would probably equal or exceed the cost of manufacturing a totally new grinding roll whereby the advantages accruing from reusing a worn grinding roll vis-a-vis replacing the latter with a new grinding roll would not be realized.
Thus, it is not surprising to find in the prior art evidence of the fact that various efforts have heretofore been undertaken directed towards achieving improvements in the wear life of pulverizer rolls. For ease of reference, it is possible to classify these efforts into three basic categories. First, there are those efforts which have been directed towards realizing improvements in the manner in which the original manufacture of the pulverizer rolls is accomplished and/or in the manner in which the pulverizer rolls are resurfaced after having become worn. Secondly, there are those efforts which have been directed towards producing an improved nickel-chromium alloy that is capable of being utilized for purposes of fabricating at a minimum therefrom the external, i.e., wear, surface of the pulverizer rolls. Thirdly, there are those efforts which have been directed towards producing an improved high chromium alloy that is capable of being utilized for purposes of fabricating at a minimum therefrom the external, i.e., wear, surface of the pulverizer rolls.
Commencing with a consideration of the first category enumerated above of those efforts that have been undertaken in an effort to improve the wear life of pulverizer rolls, reference may be had in this regard, by way of exemplification and not limitation, to U.S. Pat. No. 4,389,767, which issued on June 28, 1983, and which is assigned to the same assignee as the present application. U.S. Pat. No. 4,389,767 discloses a method of manufacturing a pulverizer roll which is characterized in that the external, i.e., wear, surface of the pulverizer roll is shaped in accordance with the predicted wear characteristics that the pulverizer roll is expected to experience based on the nature of the use to which the pulverizer roll is intended to be put such that the contour thereof replicates that of a worn pulverizer roll, and a substantially uniform layer of a material characterized by its wear-resistant qualities is applied to the external surface of the pulverizer roll so as to form the wear surface of the pulverizer roll.
A further exemplification of those prior art efforts that are deemed to be classifiable in the first category enumerated above is that which can be found set forth in U.S. patent application Ser. No. 446,850. The latter U.S. patent application bears a filing date of Dec. 6, 1982, and is assigned to the same assignee as the present application. The subject matter of U.S. patent application, Ser. No. 446,850 is directed to a pulverizer roll embodying a trimetal form of construction and wherein the core material, i.e., the first or inner layer, of the trimetal roll consists of a relatively soft material that is noted for its good machinability, the next, i.e., second or intermediate, layer of the trimetal roll comprises a material that has medium wear-resistant qualities and the last, i.e., third or outer, layer of the trimetal roll comprises a material having highly abrasive resistant qualities.
Moving on to a consideration of the second category of prior art efforts to which mention has been made hereinbefore, reference may be had for this purpose, by way of exemplification and not limitation, to U.K. Patent Application GB2,027,702A. To those skilled in this art, it has long been known to fabricate the external, i.e., wear, surface of pulverizer rolls from that material commonly referred to as Nihard. U.K. Patent Application GB2,027,702A, on the other hand, is directed to a white cast iron alloy, which the assignee of this application, i.e., Sheepbridge Equipment Limited of the United Kingdom, has seen fit to refer to as "premium Nihard". Allegedly, the latter alloy has better wear-resistant qualities than that of what will for want of a better designation be referred to herein henceforth as ordinary Nihard, i.e., that material which those skilled in this art have long referred to as Nihard. As set forth in U.K. Patent Application GB2,027,702A, the white cast iron alloy, i.e., Premium Nihard, to which this application application is directed comprises, by weight, 2.8%-3.5% Carbon, 0.6%-2.0% Silicon, 0.05 %-0.5% Manganese, 0.05%-0.25% Sulfur, 0.5%-1.5% Phosphorous, 3.5%-5.0% Nickel, 2.5%-4.5% Chromium, 0.2%-0.7% Molybdenum, and with the balance being iron and incidental impurities. Further, it is stated in U.K. Patent Application GB2,027,702A that the white cast iron alloy, which comprises the subject matter thereof, may also contain up to 0.01% Bismuth, by weight, to ensure against possible graphite formation particularly in heavy section castings.
The third category of prior art efforts to which mention has been made hereinbefore will next be considered. With respect thereto, the material, which is known as "Stoody 103", may be viewed as representing the culmination of one such effort. This material, i.e., Stoody 103, is known to be commercially available from The Stoody Company of California. insofar as concerns the composition of the Stoody 103 material, the latter is reported to include, among others, the following elements in the approximate amounts, by weight, listed: Carbon-4.0%, Manganese-5.0% and Chromium-27.0% to 28.0%. On the other hand, Stoody 103 material is said not to include either Molybdenum or Boron, except perhaps in barely discernible quantities. In the context of pulverizer rolls, Stoody 103 material is commonly employed as a weld overlay material that is applied to the outer surface of the pulverizer roll whereby the Stoody 103 material forms the external, i.e., wear, surface of the pulverizer roll. It has been alleged that the wear-resistant qualities of Stoody 103 material are better, by a factor of 11/2 to 2 times, than those of the material which has been referred to herein as ordinary Nihard.
Another example of an effort which is deemed to be classifiable into the third category, as the categories have been defined hereinbefore, of efforts that have been undertaken in the prior art is that which resulted in the development of the material, which forms the subject matter of the International Patent Application which bears the Application Number PCT/US82/00976. The latter International Patent Application is directed to an abrasive resistant white cast iron. More specifically, the white cast iron which forms the subject matter of the aforereferenced International Patent Application is defined therein as comprising a base of iron, and, by weight, 2.0% to 4.5% Carbon, 0.001% to 4% Boron and one or more of the following alloying elements: 0.001% to 30% Vanadium, Titanium, Niobium, Tantalum, Molybdenum, Nickel, Copper or Chromium, or mixtures thereof.
Continuing, it has been known in the prior art to resurface a worn grinding roll. Moreover, it is known that when the resurfacing of a worn grinding roll has taken place, this resurfacing has frequently taken the form of providing the exterior surface of the worn grinding roll with a suitably dimensioned layer of weld material. The results with some notable exceptions obtainable from the hardfacing of a grinding roll have, generally speaking, been of a satisfactoy nature.
One of these exceptions has been noted in connection with the use of one particular technique for purposes of accomplishing the resurfacing of a worn grinding roll. The technique to which reference is had here is that of bulk welding. The technique of bulk welding is a technique whereby, as those skilled in this art know, a wire, flux and a bulk metal powder are utilized in a submerged arc welding process. More specifically, in accordance with the technique of bulk welding, the bulk metal powder is fed at a controlled rate onto the base metal, the flux is piled on top of the powder, and the wire is welded through the combination melting of the powder and the flux. Note is taken here of the fact that the powder may function to alloy the weld or to just increase the deposition rate.
In order to achieve a proper weld, it is important, however, that the bulk metal powder, which is being utilized in connection with the resurfacing of the worn grinding roll, be uniform and non-segregative. If it is not, a faulty weld can be produced. To this end, one of the critical constituents in hardfacing a worn grinding roll is the carbon content. Moreover, it is known that the carbon content required for use in such applications is very fine and light, and as such tends to float on top of the other powders that are utilized in the bulk welding process. On the other hand, if the graphite is permitted to float on top of these other powders, this can give rise to an inconsistent alloy being produced, which for obvious reasons is something which is to be avoided. In addition, because of its fineness graphite has in the past demonstrated a tendency to cause the feeding device to become plugged. Further, this pluggage of the feeding device has been found to occur as a result of a bridging action on the part of the fine graphite.
One way of obviating the problem of nonuniformity of the powders, when employing the technique of bulk welding for purposes of accomplishing the resurfacing of a worn grinding roll, is to pelletize the powders whereby all of the constituents thereof are combined homogeneously into pellets. Such a pelletizing process requires the addition of a binder as well as the performance of the steps of mixing to pelletize and baking to set the binder. Some of the advantages that accrue from the use of such a pelletizing process is that there is provided a uniform screen size powder with no fines, and each pellet has essentially the same composition. In addition, such a pelletizing process is also known to permit greater flexibility in alloy composition by thus enabling different formulations of a homogeneous powder to be developed.
Another exception to the general rule that hardfacing a grinding roll produces acceptable results is when the hardfacing is attempted to be applied to certain forms of new grinding rolls. In view of the generally satisfactory experience which the prior art has had with the resurfacing of a worn grinding roll, attempts have been made to provide the exterior surface of new grinding rolls with hardfacing. Unfortunately, such attempts at hardfacing the exterior surface of a grinding roll when applied to certain forms of new grinding rolls as opposed to worn grinding rolls have not heretodate met with a great deal of success.
The impetus behind these attempts at hardfacing the exterior surface of new grinding rolls resides largely in the desire to effectively extend the operating life of new grinding rolls. A major reason why the attainment of a longer operating life for new grinding rolls is sought is that it would enable one to extend the point in time at which it becomes necessary to shut down the bowl mill for purposes of enabling the removal therefrom of the worn grinding roll and the replacement thereof with an unworn grinding roll. In this regard, it is important to note that a plurality of bowl mills are commonly employed to provide the required amount of pulverized coal to a coal-fired steam generator, and that each of these bowl mills normally embodies three grinding rolls that are each susceptible to having to be removed and replaced as they become worn. Also, there is the matter of the time and effort as well as the cost associated therewith that needs to be expended in the course of effecting such removal and replacement of a worn grinding roll. Obviously, therefore, if one were able to reduce the frequency with which grinding rolls become sufficiently worn as to require replacement, cost savings could be realized in terms of the time and effort required to be expended to effect such replacement.
A need has, thus, been shown to exist in the prior art for a new and improved material, which is characterized by its highly abrasive resistant qualities. In addition, a need has been evidenced for such a highly abrasive resistant material which, at a minimum, would be suitable for employment as the exterior, i.e., wear, surface of a grinding roll and which would in turn enable a grinding roll to be provided that would be characterized by the fact that the grinding roll possesses a significantly longer operating life than that commonly found to be obtainable from a newly manufactured grinding roll that has been manufactured in accordance with heretofore known prior art materials. To this end, such a highly abrasive resistant material should be capable of being cast. Moreover, there has been evidenced a need for such a highly abrasive resistant material which would be suitable for employment for purposes of resurfacing a worn grinding roll wherein the highly abrasive resistant material would, at a minimum, form the exterior, i.e., wear, surface of the grinding roll. Such a highly abrasive resistant material should be suitable for employment for resurfacing a worn grinding roll notwithstanding the nature of the material which had originally been utilized as the exterior, i.e., wear, surface of the grinding roll. Furthermore, such a highly abrasive resistant material should be capable of being applied to the exterior, i.e., wear, surface of either a new grinding roll or a worn grinding roll by the technique of bulk welding. As such, such a highly abrasive resistant material should be susceptible to be pelletized whereby to ensure the uniformity and non-segregation thereof when the application thereof is by the technique of bulk welding.
It is, therefore, an object of the present invention to provide a new and improved material which is characterized by its highly abrasive resistant qualities.
It is another object of the present invention to provide such a highly abrasive resistant material that is capable of being cast.
It is still another object of the present invention to provide such a highly abrasive resistant material which is particularly suited for use to form the exterior, i.e., wear, surface of a grinding roll that is designed to be employed in a bowl mill.
A further object of the present invention is to provide such a highly abrasive resistant material which is capable of being applied by the technique of bulk welding to a new grinding roll to form the exterior, i.e., wear, surface thereof.
A still further object of the present invention is to provide such a highly abrasive resistant material which is capable of being utilized to resurface the exterior, i.e., wear, surface of a worn grinding roll notwithstanding the nature of the material of which the exterior of the grinding roll had originally been formed.
Yet another object of the present invention is to provide such a highly abrasive resistant material which is capable of being pelletized for purposes of being applied by means of the technique of bulk welding to a grinding roll to form the exterior, i.e., wear, surface thereof.
Yet still another object of the present invention is to provide such a highly abrasive resistant material which is relatively inexpensive to provide, is easy to employ and is characterized by its relatively long wear life as compared to the wear life of the prior art material which has heretodate been available for use for similar purposes.